Electroless plating on nonconducting surfaces



United States Patent 3,367,792 ELECTRQLESS PLATING 0N NON- CUNDUCTENG SlJRFACEfi Charles A. Levine, Concord, Calif, assignor to The Dow Chemical Company, Midland, Mich, a corporation of Deiaware No Drawing. Filed Sept. 16, 1963, Ser. No. 309,283 13 Claims. (Ci. 117-47) This invention relates to the plating of metals on nonconducting surfaces. More particularly, this invention relates to an electroless method for the plating of nonconducting surfaces.

Common plating techniques usually require that a surface which is to be plated with metal be immersed in a plating bath and electrically connected through a direct current source to another electrode also immersed in the plating bath. Polarity of the electrical connection is usually such that the article whose surface is to be plated is cathodic, While the other electrode is anodic. The anode is usually of the same metal which is to be plated. The above described apparatus is generally referred to as a plating cell.

Obviously, an ordinary plating cell is not operable when the surface to be plated does not conduct electricity. Plating may occur on some conducting portion of the cathode assembly, but not ordinarilyon the nonconducting surface.

Various techniques have been tried for the plating of non-conducting surfaces. Among them is the method, as disclosed in U.S. Patent 2,956,900, of simultaneously spraying on the non-conducting surface a reducing solution and a solution rich in ions of the metal to be plated. This procedure requires expensive and complicated apparatus to accomplish the desired result. Further, such a procedure is wasteful since it is diilicult to adjust to equivalent amounts of solution and also since only a minor amount of the metal content of the resultant mixed solution is actually deposited on the surface to be plated.

In contrast, the present invention provides a simple, inexpensive method for the plating of non-conducting surfaces with a metal.

It has now been discovered that the plating of a non-conducting surface can be effected by positioning a second metal such as in the form of metal foil or fine mesh metal screen in close proximity to the non-conducting surface to be plated and immersing the thus formed combination in a metallic plating bath. The combination is allowed to remain in the bath until the desired thickness of coating has been deposited on the non-conducting surface. When sufiicient metal has plated on the non-conducting surface, the combination of metal foil or screen and plated non-conducting material is removed from the plating bath. When the metal foil or screen is removed from the non-conducting material, a fine continuous metallic plate is revealed on the surface of the non-conductin g material. If desired, a pattern may be plated onto the nonconducting surface by use of a corresponding foil pattern. Also a grid pattern can be achieved if a metal screen is used and the plating is stopped at an early stage.

Suitable metal foils for use in the method of the present invention are of those metals which are above the metal to be plated on the non-conducting surface in the electromotive (EMF) series, and which do not rapidly disintegrate in water. For instance, if nickel is the metal to be plated, suitable metals for the foil include aluminurn, beryllium, iron, magnesium, zinc, etc. If zinc is the metal to be deposited, aluminum, beryllium, magnesium, etc. are suitable as the foil metal. Similarly, for the deposition of copper, foils of aluminum, cadmium, cobalt, iron, lead, magnesium, nickel, tin, zinc, etc. can be used. Metals 3,367,792. Patented Feb. 6, 1968 ice pertinent to the practice of this invention have the following order in the E.M.F. series: Mg, Be, Al, Mn, Zn, Cr, Fe, Cd, Co, Ni, Sn, Pb, Mg, Cu, Ag, Pt and Au.

Solutions for use in the method of the present invention are those aqueous solutions which are generally acceptable for the standard plating bath methods. Examples of such solutions are to be found in US. Patents 2,956,900 and 3,095,309; Bull. Soc. Chem, France (1961) pp. 1180-1183; and J. Electro. Soc., vol. 104, No. 2 (February 1957) pp. 104-111. Adjustments in the pH of the plating solution may be made in accordance with general electrolytic plating techniques. Particular conditions for the plating solution and the plating operation are those common for the particular metal being used and appropriate conditions are therefore obvious to anyone skilled in the plating art.

Pressurized equipment can be used to carry out the method of the present invention at temperatures over 100 C. Generally, at higher temperatures deposition is more rapid, but the deposit may have the disadvantage of being coarse and non-uniform. At lower temperatures, deposition is generally slower, but the deposit is usually fine, uniform, and adherent. At temperatures below about 50 C., for example as low as 0 C., but advantageously at least at room temperature, deposition of metal will occur on the non-conducting surface, but the rate is extremely slow. Thus, the starting temperature range for the process of this invention is advantageously from about 50 to about 95 C., preferably -95 C. For best results, the plating is initiated at a higher temperature, and the bath temperature is subsequently lowered to the minimum operable temperature consistent with time considerations, room temperature often being desirable. By this method, an initial starting coat is rapidly formed facilitating and expediting later plating. Finishing the plating at the lower temperature gives the advantages of low temperature plating hereinbefore described.

One factor determining the time required for deposition of a desired thickness of coating is the temperature of the plating bath. In most cases in nickel plating 46 hours is sufiicient time at temperatures of 50-80 C and 2-4 hours at 8090 C.

Non-conducting materials which can be plated by the method of this invention are those which are not affected by aqueous media, such as the plating baths, to an extent that will alter their physical configuration. Various plastics, ceramics, etc. can be plated by the method of this invention. The various plastics include thermoplastic and thermosetting synthetic organic resinous materials, particularly ion exchange resinous polymeric materials.

Ion exchange resinous materials suitable for utilization in the metal plating method of the present invention generally fall within three classes. The first of these materials consists entirely of articles formed of ion exchange resin. The second of such materials consists of articles formed from a base resin having incorporated therein an ion exchange resin. The third class of such materials consists of articles formed from a grafted base resin reacted with ion exchange forming materials. Any shaped articles of the ion exchange resins known to the art may be utilized in the metal plating method of the present invention.

As is well known, such ion exchange resins contain a mobile ionic substituent. In the case of cation exchange resins, these exchangeable cation groups are generally attached to acidic groups, such as a sulfonic acid group, a. carboxyl group, and the like. This ionizable substituent is attached to a polymeric material such as phenol-aldehyde resins, polystyrene-divinylbenzene, polystyrene, styrenegrafted polyethylene, or the like. The cation component is a mobile and replaceable ion electro-statically associated with the fused component of the resin molecule. It is the ability of the cation to be replaced under appropriate conditions by other cations which imparts the ion exchange characteristics to these materials. For suitable cationic exchange materials, reference is made to Juda et al., U.S. Reissue 24,865; Johnson, U.S. 2,658,042; Ferris, U.S. 2,678,306; and Bodamer, U.S. 2,681,320. As preferred cationic exchange resins may be mentioned: (1) sulfonated polystyrene formed by sulfonating polystyrene or styrenedivinyl benzene copolymers or by forming an admixture of sulfonated polymers of styrene with other polymers, and (2) polyethylene having styrene grafted thereto by chemical and radiation means followed by reaction with chlorosulfonic acid.

Anion exchange resin materials also suitably employed for present purposes may be formed of any of such materials known to the art and are similar in their action to the cation exchange resins except that in the anion eX- change resins it is the ability of the anion to be replaced which causes the ion exchange activity. Generally speaking, such anion resins are formed by incorporating an amine group in the resin. Particularly suitable are quaternary amines. Prefered anion exchange materials suitable for use in the present invention are the following:

( 1) polystyrene resinous materials chloromethylated and reacted with a tertiary amine;

(2) polyethylene resinous materials having incorporated therein quaternary amine ion exchange beads; and

(3) polyethylene resinous materials having polystyrene grafted thereto by chemical or radiation means and reacted with chloromethyl ether, this reaction product being further reacted with triethyl amine.

For other suitable anion exchange resins, reference is made to the above-mentioned Juda patent; Kropa, U.S. 2,663,702; and Bodamer, U.S. 2,681,319.

In the practice of this invention, care should be taken to maintain sufficient space between the metal foil used and the surface to be plated so that plating solution can circulate between them. Sufficient space is provided, however, if the article to be plated is simply rolled loosely in the metal foil. Interior surfaces may be plated by inserting foil into the opening, cavity, or depression in the surface. In a preferred technique, fine mesh metal screen can be used instead of foil so that freer access of the plating solution to the area to be plated is provided. Surprisingly, smooth continuous layers of metal can be deposited in this way. Ordinary window screening, made either of aluminum or iron, having 18 openings per inch is appropriate. Obviously perforated foil is also suitable.

Small articles and interior surfaces not readily plated by the techniques of prior methods are easily plated by the process of this invention. Another advantage is that a single plating bath can be used instead of the two or more solutions as in some of the methods proposed for plating non-conducting surfaces.

The process of the present invention is best illustrated by the following examples which are set forth to illustrate, and are not intended to limit the scope of the invention nor the manner in which it can be practiced. In these examples and throughout the specification, parts and percentages unless specified otherwise are parts and percentages by weight respectively.

EXAMPLE I A solution of 3 parts NiCl .6H O, 2 parts NaI-I PO H O, 5.4 parts NH Cl and 10.6 parts Na citrate in 100 parts of water is adjusted in pH to within the range of 8-10 with NH OH. A piece of plastic film about 3 mils thick consisting of polyethylene having sulfonic acid groups on the polymeric backbone (Nalfilm II, a commercial cation exchange membrane) is covered lightly with aluminum foil. Care is taken that the foil is suiticiently loose that solution can easily get between the foil and the plastic surface. The foil covered plastic is immersed in the solution described above and the solution is heated to about C. After about two hours, metallic nickel is visible on the surface of the plastic film when the foil is folded back. As the immersion is continued, the nickel plate on the plastic surface becomes thicker. The test is halted after four hours. A thick coating is found on the plastic surface which is proved by X-Ray technique to be metallic nickel.

EXAMPLE II A solution of 3 parts NiCl .6H O, 2 parts NaH PO H 0, 5 parts NH CI, and 10 parts sodium citrate in parts of water is adjusted in pH to within the range of 8l0 with NH OH. A piece of the same type of film as used in Example I is loosely and only partially covered with aluminum foil, and the assembly thus formed is immersed in the solution which has been preheated to 80C. After about 2.5 hours of immersion at about 80 C., the solution with the foil therein is allowed to cool to room temperature. Immersion is continued at room temperature for about 16 hours, after which the film is removed from the solution. Examination of the plastic after removal from the solution shows a fine, bright nickel mirror plate on the portion of the plastic surface which had been covered with aluminum foil. The area of plastic surface not covered by aluminum foil during immersion shows no metallic deposit whatever. X-ray tests show that the mirror is nickel plate.

EXAMPLE III The procerdure of Example II is repeated a number of times using respectively, in place of the plastic used therein, sheets of polyethylene, polystyrene, polyvinyl acetate, polymethyl rnethacrylate, glass and ceramic tile. In each case the plating is similarly eifect as in Example II.

EXAMPLE IV A plating bath of nickel is prepared by making to a volume of 100 ml. with distilled water a solution of 3 grams of NiCl -6H O, 2 grams of NaH PO 5.4 grams of NH Cl and 10.6 grams sodium citrate. The pH is adjusted to 8.6 with NH OH. In a 250 ml. beaker a piece of Nafilm I is partially covered with aluminum screening having 18 openings per lineal inch and immersed in the plating solution prepared as above. After being kept at about 80 for one hour, the film is plated in a clearly discernible grid pattern with nickel. After two hours more at the same temperature a continuous deposit of nickel is formed. The resistance measured between two ohmeter probes placed one cm. apart on the plated surface is 3.5 ohms.

EXAMPLE V A sample of Nafilm is loosely wrapped with aluminum foil out in a geometric pattern. After 5 hours at about 80 in a bath as described in Example IV, the film is plated with nickel in the pattern of the foil.

While foils and screens of the various metals have been specified, thicker pieces of these metals will also work satisfactorily, particularly in interior spaces, such as inside tubes, etc. where rods of the metals can be conveniently inserted. However, in most cases foils and screens are more easily applied and involve more economical application of the metals. For ease in flexing and adapting the screening to desired shapes screening made of wire less than W inch diameter is preferred, although screens of larger diameter wire will also work satisfactorily.

Plated surfaces produced according to the practice of this invention can be used as electrodes, electrical conductors, catalytic surfaces, for manufacture of printed circuits, etc., in accordance with known procedures for making and using such products.

While certain features of this invention have been described in detail with respect to various embodiments thereof, it will, of course, be apparent that other modifications can be made within the spirit and scope of this invention and it is not intended to limit the invention to the exact details shown above except insofar as they are defined in the following claims:

The invention claimed is:

1. A process of plating a non-conducting surface with a first metal comprising the steps of positioning in close proximity to and spaced from said non-conducting surface, a second metal which is above the said first metal in the electromotive force series, immersing the composite article thus formed in a plating solution bath of said first metal, and allowing said solution to flow in contact with and between said second metal and said nonconducting surface.

2. The process of claim 1 in which said bath is maintained at a temperature of 100 C.

3. The process of claim 1 in which said bath is maintained at a temperature of 5095 C.

4. The process of claim 3 in which said second metal is aluminum.

5. A process of plating a non-conducting surface with nickel comprising the steps of loosely covering said nouconducting surface with an aluminum shaped article selected from the class consisting of aluminum screen and aluminum foil, immersing the composite article thus formed in a nickel plating bath, and allowing said solution to flow between said non-conducting surface and said aluminum shaped article.

6. The process of claim 5 in which said bath is maintained at a temperature of about 8095 C. and said composite article is retained in said plating bath for at least two hours.

7. The process of nickel plating a surface of a nonconducting synthetic organic resinous article, comprising forming a composite article consisting of said resinous article and in close proximity thereto an aluminum shaped article selected from the class consisting of aluminum foil and aluminum screening, immersing the composite article thus formed in a nickel plating bath and allowing the solution in said bath to flow between said resinous article and said aluminum article, and removing said composite article from said bath when a continuous adherent coating of nickel plate of the desired thickness has formed thereon.

8. The process of claim 7 in which said bath is maintained at a temperature of room temperature to about 95 C.

9. The process of claim 7 in which said bath is maintained at a temperature of about C. to about 95 C.

10. The process of claim 7 in which said synthetic resinous article is an ion exchange resin.

11. The process of claim 10 in which said synthetic resin article is an anion exchange resin.

12. The process of claim 10 in which said synthetic resin article is a cation exchange resin.

13. The process of claim 12 in which said cation exchange resin is a sulfonated polymer of styrene.

14. The process of claim 12 in which said cation exchange resin is a sulfonated polymer of ethylene.

15. The process of claim 9 in which said article is immersed in said bath for a period of about 2-6 hours.

16. The process of claim 7 in which said bath is maintained at a temperature of about -95 C. and said article is immersed in said bath for about 24 hours, then lowering the temperature of said bath to about room temperature and allowing said article to remain in said bath until a desired thickness of nickel plating has been deposited on said non-conducting surface.

17. The process of claim 7 in which said bath is maintained at about C. while said composite article is immersed therein for a period of about 4 hours.

18. The process of claim 7 in which said bath is maintained at about 80 C. while said composite article is immersed therein for about 2.5 hours, thereafter allowing said bath to cool to room temperature and allowing said composite article to remain in said bath at room temperature for about 16 hours.

References Cited UNITED STATES PATENTS 2,664,363 12/1953 Meth 1l7160 20,353 5/1858 Knight 204-30 X 225,186 3/1880 Waltz 204--30 925,365 6/1909 Marino 20430 3,035,944 5/1962 Sher et al. ll7-47 X 3,222,218 12/1965 Beltzer et a1. 117-47 X RALPH S. KENDALL, Primary Examiner.

ALFRED L. LEAVITT, Examiner. 

1. A PROCESS OF PLATING A NON-CONDUCTING SURFACE WITH A FIRST METAL COMPRISING THE STEPS OF POSITIONING IN CLOSE PROXIMITY TO AND SPACED FROM SAID NON-CONDUCTING SURFACE, A SECOND METAL WHICH IS ABOVE THE SAID FIRST METAL IN THE ELECTROMOTIVE FORCE SERIES, IMMERSING THE COMPOSITE ARTICLE THUS FORMED IN A PLATING SOLUTION BATH OF SAID FIRST METAL, AND ALLOWING SAID SOLUTION TO FLOW IN CONTACT WITH AND BETWEEN SAID SECOND METAL AND SAID NONCONDUCTING SURFACE. 